As regards rate-reduction audio encoding, the audio signal often must undergo a bandpass limitation when the bit rate becomes low. This bandpass restriction is necessary to preclude introducing audible quantizing noise into the encoded signal. In such a case the high-frequency content of the original signal should be regenerated to the extent possible.
It is known from the state of the art, and in particular from the patent document WO 9,857,436 A, to regenerate the high-frequency special content of the original signal by harmonically transposing the low-frequency spectrum of the decoded signal toward the high frequencies. This transposition is carried out by recopying the spectral value of a fundamental fk at all frequencies of the harmonic series n*fk. The shape of the high-frequency spectrum so developed is adjusted by applying spectral weighting factors.
FIG. 1 schematically illustrates the spectral reconstruction apparatus of the state of the art. The encoded audio signal is decoded by a decoder 101 that applies a low-frequency spectrum signal SB to a bank 102 of analyzing filters, the outputs k of these filters being connected to the inputs of harmonic orders n*k (n=1 . . . N) of a set of synthesizing filters 104 after having been weighted by spectral weighting factors 103. For simplicity, the decimators at the output of the analyzing filter bank (respectively the interpolations of the synthesizing filter bench) were omitted.
The synthesized signal SH exhibits a high frequency spectrum. It is added to the signal SR by a summer 105 to generate a reconstructed wideband signal SR.
The above cited reconstruction technique is based on a sub-band analysis and on a complex harmonic duplication. It entails computationally expensive methods for adjusting phase and amplitude. Moreover the spectral weighting factors only coarsely model the spectral envelope. In general and outside any decoding context, it is important that it be feasible to enhance the spectral content of a physical signal exhibiting an incomplete spectrum. The term “incomplete spectrum” denotes any spectrum with limited support or any spectrum exhibiting “holes”. Such is the case in particular as regards an audio signal or a speech signal with limited bandpass: spectral enhancement then shall substantially improve sound quality and signal intelligibility.